Taking methotrexate and aspirin together is classified as a major interaction by the FDA-sourced information available through drug labeling databases. Aspirin, an NSAID (nonsteroidal anti-inflammatory drug), can increase the amount of methotrexate in your bloodstream, raising the risk of serious side effects and potentially affecting how well methotrexate works for you. This is not a combination to use without explicit guidance from your pharmacist or physician.
The interaction record drawn from FDA drug labeling designates this pair as a major severity interaction. According to the source data, aspirin is an NSAID that may increase methotrexate plasma concentrations—meaning the drug builds up to higher levels in your blood—which increases the risk of severe adverse reactions. Additionally, the documented interaction notes that this elevation in methotrexate exposure may reduce clinical effectiveness, meaning the drug might not work as well as intended for your condition.
This information comes from NLP analysis of methotrexate labeling and is part of the standard FDA-curated drug interaction knowledge base. The record was last updated in May 2026, reflecting current labeling guidance.
To understand what's happening, it helps to know a little about how both drugs move through your body.
What methotrexate is and how it works: Methotrexate is a disease-modifying antirheumatic drug (DMARD) used to treat conditions like rheumatoid arthritis, certain cancers, and autoimmune diseases. It works by suppressing an overactive immune system or slowing the growth of certain cells. Because methotrexate is powerful and can be toxic at high concentrations, the dose you take is carefully calculated to be therapeutic—strong enough to help your condition but not so strong that it causes organ damage or severe blood cell problems.
What aspirin does to methotrexate: The source record states that aspirin increases methotrexate plasma concentrations—in other words, it slows how quickly your body clears methotrexate from your blood. When methotrexate levels rise unexpectedly, you face two risks: first, the higher concentration can cause serious toxicity (such as bone marrow suppression, liver damage, or kidney injury), and second, the paradoxical risk that the drug becomes less effective at treating your underlying condition, even though more of it is in your system.
Why this happens—the documented mechanism: The source record indicates that aspirin increases methotrexate exposure without detailing the precise molecular mechanism in the record itself. Aspirin and other NSAIDs are known to affect kidney function and can reduce the kidney's ability to filter and eliminate methotrexate. NSAIDs may also compete with methotrexate for protein binding in the bloodstream, potentially freeing up more active methotrexate. However, the key point from the source is that the interaction is documented and major, regardless of which of these mechanisms (or combination thereof) is at play in your individual case.
Why "potentially reducing clinical effectiveness" matters: You might wonder how a drug can cause toxicity and be less effective at the same time. This can occur because the relationship between drug dose and therapeutic benefit is not always linear. In some cases, when methotrexate levels become too high, the body's response to the drug can become dysregulated or the drug can cause side effects that outweigh its benefits, effectively undermining the treatment goal.
Anyone taking methotrexate for any reason should be aware of this interaction before considering aspirin use. This includes people being treated for:
If you have kidney disease, liver disease, or reduced bone marrow function, the risks may be amplified, because your body is already working harder to clear drugs and manage their effects. Similarly, if you are taking other medications that also affect kidney function or interact with methotrexate, the combination of risks rises. However, the documented interaction applies to anyone on methotrexate who is considering aspirin, not just those with additional medical complexity.
Example 1 (illustrating methotrexate accumulation): Imagine you have rheumatoid arthritis and your rheumatologist has carefully calibrated your methotrexate dose at 15 mg per week based on your kidney function, liver function, and response to the drug. You develop a headache and, without checking your medication list, take two aspirin tablets. Over the next few days, aspirin begins to slow methotrexate clearance. The same dose of methotrexate that was safe and effective last week now accumulates to a higher level in your blood. A few days later, you notice unusual bruising, a sore throat, and fatigue—signs that your bone marrow is being affected. You contact your doctor and learn that methotrexate toxicity is suspected. This scenario illustrates why checking before adding any over-the-counter pain reliever is critical.
Example 2 (illustrating the effectiveness paradox): Suppose you've been stable on methotrexate for inflammatory bowel disease. You experience a flare of joint pain (a common comorbidity) and decide to use aspirin for pain relief without consulting your pharmacist. Over the next two weeks, your methotrexate level creeps up due to reduced kidney clearance. At the same time, your inflammatory bowel symptoms worsen or fail to improve, even though methotrexate is supposedly in your system at a higher concentration. This illustrates how the interaction can simultaneously increase toxicity risk and undermine therapeutic benefit.
Before you take aspirin—whether over-the-counter or as a low-dose heart-protective dose—while on methotrexate, you should have a clear conversation with your pharmacist or doctor. Here are key questions to ask:
Do not stop taking methotrexate or aspirin (if you have a medical reason for it) without speaking to your doctor. Do not self-treat pain or fever with over-the-counter NSAIDs without confirming with a pharmacist first. Your healthcare team can review your complete medication list and medical history to make a safe, individualized plan.
If you are taking methotrexate and aspirin or considering adding either drug to your regimen, use checkdruginteractions.com to review your complete medication list for documented interactions. Always confirm any medication-related decisions with your pharmacist or physician. Your healthcare team has access to your full medical history and can provide personalized guidance that this educational article cannot replace.
CDI checks every pair across up to 20 drugs — backed by FDA and NIH data.
Drug interaction data sourced from U.S. FDA drug labeling via openFDA and the U.S. National Library of Medicine (NLM), National Institutes of Health. For informational purposes only. Always consult your pharmacist or physician before making any medication decisions.
Amiodarone is a major-severity drug interaction with warfarin. Learn how it affects your blood thinner and what monitoring means.
Metronidazole increases warfarin's blood-thinning effect by inhibiting CYP2C9. This is a major interaction requiring close INR monitoring.
Gemfibrozil is contraindicated with repaglinide due to an 8.1-fold increase in drug exposure via CYP2C8 inhibition. Learn what this means...